The present invention broadly relates to the recovery of organic compounds that contaminate industrial gas streams and more particularly to an improved recovery scheme that operates with humid contaminated gas streams.
The widespread use of solvents in industrial applications has resulted in increased emissions of volatile organic compounds (VOCs) into the atmosphere, giving rise to environmental concerns and prompting stricter legislative controls on such emissions. As a consequence, manufactures of pharmaceuticals, coated products, textiles, and polymer composites and foams, as well as hydrocarbon producers and distributors, face a dilemma in removing VOCs from process gas streams in that, owing to rising energy prices, recovery costs are very often higher than the value of the VOCs recovered, even in light of rising solvent prices. This dilemma has led to inquiries into more profitable methods of recovering condensable organic vapors from process gas streams.
Conventional adsorption systems for solvent recovery from humid air typically are operated until the solvent concentration in the outlet gas stream reaches a detectable preset breakthrough level whereupon the gas flow to the adsorber is stopped. The adsorbent bed then contains solvent, other condensible organic contaminants, and some amount of water which depends on the inlet relative humidity of the solvent laden gas stream. At this point, present-day techniques involve the introduction of steam, either saturated or superheated, which displaces the solvent from the adsorbent to produce a solvent/water mixture upon condensation.
More recent technology for regenerating and recovering solvent from adsorbent beds involves the use of inert gases (though for some solvents, air also can be used) and low temperature condensation of the solvent from the regenerating gas. The low temperature required for solvent condensation at high efficiency results in water freezing in some part of the system. To avoid ice formation, various water selective dryers have been employed for drying the air stream while permitting passage of the solvent laden air for low temperature condensation, such as achieved by either the Rankin and/or Brayton cycle processes (see U.S. Pat. No. 4,480,393, for example). However, the use of such dryers increases both the capital costs of the installation and the time required for the completion of the regeneration step, as well as the regeneration of the dryer can result in solvent releases.
Also, for continuous processes, typically two adsorber beds are used, where one is adsorbing while the other bed undergoes regeneration. Any increase in the time required for the regeneration of the first bed requires additional adsorption time on the adsorbing bed to assure that the second bed still efficiently adsorbs while the first bed is being regenerated. Therefore, any time increase in the regeneration of the first bed requires a larger second bed, and vice versa.